The precise measurement of acceleration is a vital requirement in the navigation of land, air, and space vehicles. The exact position of any moving body can be calculated once the acceleration of that body and its starting position in three dimensions is known. The measurement of small acceleration forces (sometimes referred to as "micro-g") can be used to detect vibrations, oscillations, fracturing, cracking shifting, or movement in a wide range of materials. Numerous acceleration measurement applications exist in the fields of seismology, oil drilling bit angle control, oil pipeline integrity monitoring, robotic vehicle control, aircraft navigation systems, automotive anti-skid control, automotive air-bag deployment controls, motion sensors, oceanographic instrumentation, general purpose low-cost navigation systems, shipboard instrumentation, spacecraft control, and industrial control systems.
Accelerometers can be described as open loop, or closed loop. In open loop devices, no feedback or error signals are employed in the measurement function or in control of the proof mass. If a feedback signal representing the motion of the proof mass (an object of known weight whose motion is detected) is introduced to a position control element, then the device can be considered closed loop or servo controlled. This application is directed to closed loop accelerometers.
Several inherent limitations exist with conventional proof mass closed loop accelerometers. Measurement of very small accelerations require that the proof mass be allowed to move with minimum interference or friction from its mounting. In relative terms, a heavy proof mass and lightweight suspension or mounting provides for high sensitivity to small (usually less than 10 g-forces) acceleration. Therefore, very sensitive conventional accelerometers are poorly protected from large shock impulses, giving them a limited acceleration measurement range, and make them susceptible to serious damage from shock or vibration.
If the mechanical suspension system of a conventional accelerometer involves a spring, flexure, vane, or other element which can be characterized by a spring constant, then the natural frequency of the spring/mass system determines the frequency response of the accelerometer. For this reason, accelerometer designers routinely must tradeoff performance parameters, such as frequency response vs. ruggedness, and accuracy vs. range.
Many high performance accelerometers contain delicate mechanical components such as precision flexures, jewelled pivots, and gimbals. Use of these components result in the cost of the accelerometer being quite high, and the fragility great.
Another performance variable which affects accelerometers called "cross-axis sensitivity" occurs when a force is applied to an accelerometer in its non-sensitive axis, but results in a false reading or error in the sensitive axis measurement. This can occur in many accelerometers employing spring, coil, reed, vane, or flexure suspensions in which the proof mass motion is not adequately constrained to its sensitive axis.